The outer-membrane Opa proteins have a common structural motif. They form an eight-stranded ?-barrel in the outer-membrane with four surface exposed loops. Three of the exposed loops consist of variable sequence domains (termed SV, HV-1 and HV-2) while a fourth loop, near the C-terminus of the protein, is highly conserved. Opa proteins serve as bacterial adhesins. Host- encoded receptors have been characterized for certain Opa proteins in well studied laboratory strains but little is known about the adherence properties of Opa proteins in general. The contribution of conserved versus variable sequence domains to the adherence mechanism is also largely unknown. Our research objective is to characterize the behaviour of organisms expressing Opa proteins to determine whether the variable proteins represent true "antigenic variants" (i.e. have the same biological phenotype and variable sequences to mask their recognition by the immune system) or are functional variants whose expression represents an adaptive response. A. Adherence to Glycosaminoglycan Receptors: Characterization of the important Opa-protein domains in HS- receptor binding were studied using recombinant Opa proteins expressed from genetically modified opa genes in N. gonorrhoeae. The OpaA protein of Ng MS11mk was expressed as a truncated outer- membrane protein by specifically deleting each of the four surface-exposed loops. The biological activities of each of these strains was determined by measuring the binding of heparin, and purified HS-receptor (gift of Dr. J. van Putten) and determining the adherence and internalization capabilities of the modified strains using Chang cells. Deletion of the HV-1 domain completely abolished adherence and internalization and drastically reduced binding of the cellular receptor. Chimeric Opa proteins were created by exchanging the loop domains of OpaA with the corresponding loops from OpaB (no adherence or internalization by Chang cells) and OpaC (adherence but no internalization by Chang cells). These results confirmed and supported the deletion analysis in that binding of the HS- receptor molecule was associated with the presence of HV-1 of OpaA. Moving the HV-1 of OpaA into an OpaB protein background resulted in a chimeric Opa protein which supported adherence and internalization of expressing cells, albeit to a level approximately half that of the native OpaA. B. Adherence to CD66 Receptors: We studied the adherence and internalization of Ng strains individually expressing each of the Opa proteins to HeLa cell lines stably transfected with five different members of the CD66 family and found differential binding of Opa variants to CD66 receptors. The basis for differential binding of Opa proteins by CD66 family members was studied using modified and recombinant versions of CD66. Modified glycoforms and fully deglycosylated CD66e molecules are recognized by Ng in an Opa-specific mannner, indicating that Opa proteins bind the peptide component of the receptor molecule. Recombinant N-terminal domains of CD66 receptors were expressed in E. coli and were bound differentially by Opa variants. The binding specificities were virtually identical to the adherence specificities of Opa variants to HeLa cell expressing native CD66 molecules. Recombinant, chimeric CD66e (binds all Opas)/CD66b (binds no Opas) molecules identified specific binding motifs that determined the receptor specificities of the CD66 family for different Opa variants